Implantable heart stimulating device, a system including such a device and a manner of using the system

ABSTRACT

In an implantable heart stimulating device, system and method, electrical stimulation pulses are delivered to first and second ventricles of a heart with a variable time interval therebetween, and signals are sensed at two different positions in the heart, from which an impedance value is derived. A minimum value and a maximum value of the impedance value are determined during a heart cycle, and a relationship between the minimum and maximum values also is determined. The time interval is varied and the relationship is monitored over a number of heart cycles. The time interval is set so that the relationship satisfies a predetermined requirement.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an implantable heart stimulatingdevice, a system including such a device and a method for heartstimulation using the system. More precisely, the invention concerns astimulating device of the type having a housing and a control circuitarranged in the housing. The heart stimulating device is designed suchthat it can be used to stimulate both ventricles of a heart.

2. Description of the Prior Art

Most heart stimulating devices, or pacers, are designed to stimulate theright ventricle of the heart. It is also known to stimulate the leftventricle. In particular for the treatment of congestive heart failure(CHF) or other severe cardiac failures, it is known to stimulate theleft ventricle, or both ventricles, in order to optimize the hemodynamicperformance of the heart.

U.S. Pat. No. 5,720,768 describes different possible electrode positionsin order to stimulate or sense the different chambers of the heart.

In different kinds of heart stimulating devices it is also known to usean impedance value in order to control different pacing parameters.

U.S. Pat. No. 5,154,171 describes the use of impedance values to controlthe pacing rate. The pacer described in this document is designed onlyto stimulate the right side of the heart.

U.S. Pat. No. 6,070,100 describes that electrodes may be positioned inboth the left atrium and the right atrium as well in the left and theright ventricles. The patent describes the possibility of sensing theimpedance between different electrodes. The sensed impedance values maybe used to improve the cardiac output.

United States Patent Publication No. 2001/0012953 describesbi-ventricular pacing. An impedance may be measured between electrodeson the right and the left sides of the heart. The variation of theimpedance with time is detected. The detected impedance variation may beused in order to synchronize the contraction of the ventricles.

United States Patent Publication No. describes different manners ofusing the proximal and distal electrodes of different leads in order toinject a current and to measure an impedance. The measured impedancevalue may be used in order to maximize the cardiac flow.

For a patient suffering from congestive heart failure (CHF) it is of agreat benefit to be able to increase the cardiac output, therebydecreasing the degree of CHF. One cause of CHF is that the left andright ventricles are not synchronized with each other. By optimising thesynchronization between the ventricles, the filling of the ventriclesand the cardiac output may be increased.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an implantable heartstimulating device which is able to deliver stimulating pulses to boththe ventricles of a heart and which is able to control the delivery ofthe stimulating pulses such that the cardiac output is improved. Afurther object is to provide such a device which uses an impedancemeasurement in order to control the delivery of the stimulating pulses.A further object is to provide such a device which is able to improvethe heart condition for a patient suffering from CHF. A still furtherobject is to provide such a device which automatically finds an optimaltime interval between stimulating pulses to the two ventricles. Anotherobject is to provide such a device which in a relatively simple manneris able to automatically deliver the stimulating pulses in an optimalway. According to the invention, also a system including such a deviceand a method for stimulating about using the system are provided.

The above objects are achieved by an implantable heart stimulatingdevice having a housing a control circuit in the housing, the controlcircuit being adapted to be connected to a first electrode to bepositioned to stimulate a first ventricle of the heart, and the controlcircuit also being adapted to be connected to at least a secondelectrode to be positioned to stimulate the second ventricle of theheart. The control circuit controls delivery of electrical stimulatingpulses to the first and second electrodes in order to stimulate thefirst and second ventricles, respectively, with the delivery of thestimulating pulses to the first and second electrodes occurring withinthe same cycle of the heart such that there is a time interval betweenthem, with the time interval being variable. Sensing circuitry sensessignals received from electrodes positionable at two different positionsin the heart. The control circuit derives an impedance value based onthe sensed signals, the impedance value being indicative of theimpedance between the electrodes positionable at two different positionsin the heart. The control circuit determines a minimum value and amaximum value of the impedance value during a heart cycle, anddetermines a relationship between the minimum and maximum values. Thecontrol circuit varies the time interval and monitors the relationshipover a number of heart cycles, and sets the time interval such that therelationship satisfies a predetermined requirement.

It should be noted that the time interval may be chosen to be equal tozero, i.e. in this case stimulation signals are delivered simultaneouslyto the first and second electrodes. Furthermore, it should be noted thatthe time interval may be positive or negative, i.e. the first electrodemay emit stimulating pulses before or after the second electrode. In apractical use of the device, the sensed signals may be derived fromelectrodes positioned on different sides of the left ventricle. Bymonitoring the impedance value between such electrodes, an indication ofthe volume of blood in the left ventricle may be obtained. The minimumand maximum values depend on the maximum and minimum, respectively, ofthe amount of blood in the ventricle. By determining the relationshipand by setting the time interval such that the relationship satisfies apredetermined requirement, the delivery of the stimulating pulses to thefirst and second electrodes can be optimized in order to improve thecardiac output. In this manner, for example, the heart condition of apatient suffering from CHF may be improved.

In a preferred embodiment of the invention, the control circuitdetermines the ratio between the minimum value and said maximum value asthe aforementioned relationship. If the electrodes between which theimpedance value is derived are suitably positioned, the ratio is closelylinked to the so-called ejection fraction (EF). It has been found to beadvantageous to use this ratio for controlling the delivery of thestimulating pulses.

In another embodiment of the invention, the control circuit determinesthe difference between the minimum value and said maximum value. Alsothis difference can function as an indication of the cardiac output andmay therefore also be advantageously used for controlling the deliveryof the stimulating pulses.

In a further embodiment of the invention, the control circuit determinesthe ratio between the minimum value and the difference between saidminimum value and the maximum value as the aforementioned relationship.This particular ratio is even closer linked to the EF if the electrodesbetween which the impedance value is derived are suitably positioned inrelation to the heart. It should be noted that preferably this ratiorefers to the absolute value of the division between the minimum valueand difference, since if the difference is negative the ratio wouldotherwise be negative. It is therefore preferably the magnitude of theratio employed.

In a further embodiment of the invention, the predetermined requirementis that the ratio is minimized. By minimizing the ratio it has beenfound that an optimal cardiac output can be achieved. It should be notedthat minimizing the ratio is of course the same as maximising theinverse of the ratio. This possibility is thus included in thedefinition of minimizing the ratio as used herein.

In a further embodiment of the invention, the control circuit operatessuch that the time interval is changed in a first direction, the firstdirection being either an increase or a decrease of the time intervaland the control circuit monitors the change of the ratio when said timeinterval is changed in said first direction. If the ratio decreases, thetime interval is further changed in the first direction until thepredetermined requirement has been satisfied. This has been found to bean advantageous embodiment for finding the time interval at which thepredetermined requirement is satisfied. For example, in this manner theminimum of the aforementioned ratio may be established.

In a further embodiment of the invention, the control circuit operatessuch that if the ratio increases, the time interval is changed in theopposite direction to the first direction, whereafter the interval isfurther changed in said opposite direction until the predeterminedrequirement has been established. In this embodiment, it is establishedthat the time interval is changed in the correct direction such that thepredetermined requirement may be satisfied in an efficient manner.

In another embodiment of the invention, the control circuit operates toenable the reception of signals indicating the activity level of aliving being into which the heart stimulating device is implanted, andthe control circuit is arranged such that the varying and setting of thetime interval are performed at a time when the signals indicate a lowlevel of activity. Preferably, the optimal time interval thus isestablished while the patient in question is at rest. This is madepossible by this embodiment, according to which the control circuit isalso able to detect the level of activity of the living being inquestion.

In a further embodiment of the invention, the control circuit operatesto enable the delivery of stimulating pulses in which at least oneatrio-ventricular delay is controllable, and the control circuit keepsthe time interval at the set value, and to: varies the atrio-ventriculardelay and monitors the relationship over a number of heart cycles, andsets the atrio-ventricular delay such that the relationship satisfiesthe predetermined requirement. With the device operating in this manner,the output of a heart can be further improved.

In another embodiment of the invention, the control circuit is arrangedto be connected to a first lead carrying the first electrode and asecond lead carrying the second electrode. The sensing circuitryoperates such that the sensed signals, receivable from electrodespositioned at two different positions, are received via these first andsecond leads, respectively. The heart stimulating device thus may beprovided with a connector portion via which the control circuit may beconnected to two different leads. Such leads may be positioned atdifferent positions in relation to the heart. It is thereby possible toderive the impedance value between selected positions.

The above objects of the invention also are achieved by a heartstimulating system having a heart stimulating device according to any ofthe preceding embodiments and a first lead carrying at least the firstelectrode and a second lead carrying at least the second electrode,wherein said first and second leads are connected to the heartstimulating device such that said first and second electrodes areconnected to the control circuit. The system thus includes the two leadsconnected to the heart stimulating device. With such a system the aboveadvantages are achieved.

According to a preferred embodiment of the system, the system operatessuch that the impedance value is sensed between an electrode of thefirst lead and an electrode of the second lead. The leads may bepositioned at suitable positions in relation to the heart. A suitableimpedance value thus can be derived between electrodes of the first andsecond leads, respectively.

The objects of the invention also are achieved by a method forstimulating a heart using a heart stimulating system according to any ofthe above embodiments, wherein the first electrode is positioned tostimulate a first ventricle of a heart of a human or animal and thesecond electrode is positioned to stimulate the second ventricle of theheart. According to this method, the device is thus actually used tostimulate the two ventricles of a heart. In this method, the advantagesdescribed above in connection with the device are achieved.

According to a preferred manner of the method, the impedance value issensed between two electrodes positioned such that the impedance valueis measured across at least a part of one of said first and secondventricles. In this method of using the system, an indication of theamount of blood in the ventricle can be derived by the impedance value.The variation of this impedance value can be used as an indication ofthe amount of blood pumped by the ventricle.

In a preferred method of using the system, the ventricle, across whichthe impedance value is measured, is the left ventricle of the heart. Tomonitor the ejection fraction of the left ventricle is particularlyimportant, for example when treating a patient suffering from CHF.

In a further method of using the system, an impedance value is sensedacross at least a part of the left atrium of the heart. Also thevariation in the amount of blood in the left atrium may be used tocontrol the heart stimulating device.

DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates a heart stimulating device according tothe invention connected to leads with electrodes positioned in a heart.

FIG. 2 is a flow chart of the function of the heart stimulating deviceaccording to one embodiment of the invention.

FIG. 3 schematically shows the variation of the impedance with time.

FIG. 4 schematically shows how a relationship between Z_(min) andZ_(max) depends on a time interval ΔT.

FIG. 5 is a flow chart of the function of the heart stimulating deviceaccording to an embodiment of the invention for adjusting the AV-delay.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

An embodiment of the invention will now first be described withreference to FIG. 1. FIG. 1 thus schematically shows an implantableheart stimulating device 10. The heart stimulating device 10 ishereinafter also called a pacer. Such a heart stimulating device 10 iswell known to a person skilled in the art and will therefore not bedescribed in all its details here. The pacer 10 has a housing 12. Acontrol circuit 14 is arranged in the housing 12. The pacer 10 includesa connector portion 16 to which a number of leads 30, 40, 50, 60 may beattached.

A first lead 30 has a distal electrode 31 (also called tip electrode)and a proximal electrode 32 (also called ring electrode). In the shownembodiment the lead 30 is thus bipolar, however, it is also possible forone or more leads to be unipolar, i.e. it has only one electrode. Thelead 30 includes electrical conductors (not shown) through which theelectrodes 31, 32 are connected to the control circuit 14. The controlcircuit 14 is also adapted to be connected to a second lead 40, whichhas corresponding electrode surfaces 41, 42.

The pacer 10 also may be arranged such that it is connectable to furtherleads. FIG. 1 shows a third lead 50 with electrode surfaces 51, 52 and afourth lead 60 with electrode surfaces 61, 62.

The control circuit 14 includes a pulse generator 14 a emits stimulatingpulses to different electrodes and sensing circuitry 14 b that sensessignals received from the electrodes. The details of the control circuit14 to perform the emission of pulses and the sensing are known to thoseskilled in the art and therefore need not be shown in more detail here.

FIG. 1 also schematically shows a heart having a right atrium RA, aright ventricle RV, a left atrium LA and a left ventricle LV. In theillustrated embodiment the electrodes 31, 32 are positioned in aconventional manner near the apex of the right ventricle RV. The lead 40is positioned such that the electrodes 41, 42 may be used for emittingstimulating pulses to the left ventricle LV. The lead 40 may for examplebe introduced through the right atrium RA, via the coronary sinus intothe middle or great cardiac vein. In the shown embodiment a third lead50 is introduced such that the electrodes 51, 52 are positioned in thecoronary sinus, a fourth lead 60 is introduced such that the electrodes61, 62 are positioned in the right atrium RA in a conventional manner.

The control circuit 14 operates such that it can deliver stimulatingpulses to both ventricles RV, LV, for example to the electrodes 31 and41. The control circuit 14 is thus operates such that stimulating pulsesto the electrodes 31, 41 can be delivered within the same cycle of theheart (within the same heartbeat) with a time interval ΔT between thepulses to the electrodes 31 and 41. The control circuit 14 operates suchthat this time interval ΔT is variable. The time interval ΔT issometimes also called the VV-interval. The control circuit 14 alsooperates such that it sense signals receivable from the differentelectrodes and such that an impedance value Z is derived based on sensedsignals, the impedance value Z being indicative of the impedance betweenelectrodes at two different positions of the heart. For example, theimpedance may be measured between the electrodes 31, 32 of the firstlead 30 and the electrodes 41, 42 of the second lead 40. The impedancevalue Z can be derived in different manners described in, for example,the above-cited documents. According to a preferred embodiment, acurrent is injected between electrodes 31 and 41 and the impedance valueis measured between the ring electrodes 32, 42. It should be noted thatit is also possible to derive an impedance value Z between otherelectrodes, for example between the electrodes 31, 32 of the first lead30 and the electrodes 51, 52 of the third lead 50. Another possibleimpedance value is the impedance between the electrodes 51, 52 of thethird lead 50 and the electrodes 61, 62 of the fourth lead 60.

The control circuit 14 operates such that it can determine a minimumvalue Z_(min) and a maximum value Z_(max) of the impedance during aheart cycle. Furthermore, the control circuit 14 determines arelationship between Z_(min) and Z_(max). The control circuit 14 variesthe time interval ΔT and monitors the relationship over a number ofheart cycles. Moreover, the control circuit 14 operates to set the timeinterval ΔT such that the relationship satisfies a predeterminedrequirement.

The pacer 10 also can be arranged to receive signals indicating theactivity level of a living being into which the heart stimulating device10 is implanted. Such signals, for example, can be produced by anactivity sensor 18 included within the housing 12. Different kinds ofactivity sensors 18 are known to those skilled in the art. For example,such an activity sensor can sense the movement of the pacer 10 andthereby the movement of a being in which the pacer 10 is implanted. Itis also possible to detect the activity of the patient by sensingsignals received from different electrodes connected to the pacer 10.

The impedance Z measured between, for example, the electrodes 32 and 42depends on the amount of blood in the left ventricle LV. As the amountof blood in the ventricle LV varies during a heart cycle, the measuredimpedance value Z also varies. FIG. 3 illustrates schematically how theimpedance value Z may vary with time t over a heart cycle HC. Theimpedance value Z is low when the ventricle LV is filled with blood.During the systolic phase, when the ventricle LV pumps out the blood,the impedance Z increases to a maximum value Z_(max), whereafter theimpedance value Z is lowered when the ventricle LV fills with bloodduring the diastolic phase. The control circuit 14 is thus determinesZ_(min) and Z_(max) during a heart cycle may be determined. By sensingevents in the heart, the control circuit 14 can distinguish differentheart cycles from each other.

The difference Z_(min)−Z_(max) is related to the stroke volume of theventricle LV. The so-called ejection fraction EF is defined as thestroke volume divided by the end diastolic volume. The ejection fractionEF is thus related to (Z_(min)−Z_(max))/Z_(min). Preferably, we maydefine the ejection fraction EF as the absolute value in order to alwaysget a value that is larger than zero. The ejection fraction EF is alsorelated to the value of Z_(max)/Z_(min).

The control circuit 14 determines a relationship between Z_(min) andZ_(max). This relationship, for example, can be any of theabove-described relationships which relate to the stroke volume or theejection fraction EF. One example of this relationship is the ratio ofZ_(min)/Z_(max). The control circuit can be arranged to determine thetime interval ΔT such that the ratio Z_(min)/Z_(max) is minimized (orsuch that its inverse is maximized). The control circuit 14 is thus setsthe time interval ΔT such that a predetermined requirement is satisfied.In this example, the predetermined requirement is thus thatZ_(min)/Z_(max) is minimized. By setting ΔT such that the predeterminedrequirement is satisfied, the stroke volume, or the ejection fractionEF, is controlled to be as large as possible. The cardiac outputtherefore is improved. To improve the cardiac output in this manner isimportant, for example, for a patient suffering from CHF.

The flow chart of FIG. 2 illustrates how the control circuit 14 canoperate. The flow chart starts by choosing a value for ΔT. This value,for example, may be a previously set value for ΔT or that ΔT=0.Stimulating pulses are thereafter delivered to the electrodes (forexample to the electrodes 31 and 41) with the time interval ΔT. Theimpedance value Z is sensed over at least a heart cycle. Z_(min) andZ_(max) are determined. In order to improve the measurement of Z_(min)and Z_(max) it is also possible to sense the impedance variation duringseveral heart cycles before determining Z_(min) and Z_(max). Z_(min) andZ_(max) may in this case be the average value of Z_(min) and Z_(max),respectively, over several heart cycles.

Thereafter a relationship between Z_(min) and Z_(max) is determined. Aspointed out above, this relationship may for example be Z_(min)/Z_(max).The value of the determined relationship is stored. A new value for ΔTis set and the previous steps are carried out again in order todetermine a new relationship between Z_(min) and Z_(max) and to storealso this relationship. The new value for ΔT may be chosen in differentmanners. It is possible, for example, to change ΔT in a first direction,for example to increase ΔT with a small amount. The control circuit 14then monitors whether the relationship Z_(min)/Z_(max) increases ordecreases. If this relationship decreases, the time interval ΔT may befurther increased and the steps are carried out again in order todetermine another relationship between Z_(min) and Z_(max). If ratioZ_(min) Z_(max) were to increase, the time interval ΔT may be changed inthe other direction, i.e. according to this example ΔT would then bedecreased instead of increased. It should be noted that ΔT can even benegative if it is further decreased. Whether ΔT is positive or negativeis thus decisive of which of the two ventricles is first stimulated. Theloop illustrated in FIG. 2 is carried out a sufficient number of timessuch that it is possible to determine a ΔT for which the ratioZ_(min)/Z_(max) is as low as possible.

In FIG. 4 the ratio Z_(min)/Z_(max) as a function of ΔT is illustratedwith a number of points. These points are thus obtained in the mannerillustrated in FIG. 2 by determining the relationship Z_(min)/Z_(max)for different values of ΔT. The dots illustrated in FIG. 4 define acurve in which a minimum can be established. This minimum in FIG. 4 isobtained for ΔT=T1. The control circuit 14 thus determines that atΔT=T1, the ratio Z_(min) /Z_(max) satisfies the predeterminedrequirement, i.e. in this example that the ratio is as small aspossible. When this has been decided, ΔT is set to be equal to T1.According to a preferred embodiment, the steps illustrated in FIG. 2 arecarried out when the being into which the pacer 10 is implanted is atrest. This can be done, for example, at night when the person or animalin question is asleep. As stated above, the pacer 10 may detect theactivity level. The control circuit 14 thus may operate such that thedifferent steps are carried out when a signal indicates a low level ofactivity.

Once the time interval ΔT has been determined and set equal to T1, thecontrol circuit 14 may carry out a similar determination in order tooptimise a value for AV. AV is the so-called AV-delay. This is the timebetween an atrial sensed or paced event and the delivery of aventricular output pulse. The AV-delay considered here may be, forexample, the AV-delay for the right part of the heart. As is shown inFIG. 5, a certain value for AV is chosen. Thereafter stimulating pulsesare delivered with this AV-delay and with the already set time intervalΔT=T1. An impedance value Z is sensed. This impedance value Z can be thesame impedance value as illustrated above. Z_(min) and Z_(max) aredetermined. A relationship between Z_(min) and Z_(max) is alsodetermined, for example any of the above mentioned relationships. Thisrelationship is stored. A new AV-delay is chosen. This can be done, forexample, in an analogous manner to that according to which the new ΔTwas chosen above. The previous steps are then carried out again and anew relationship is determined for the new AV-delay. The differentstored relationships are considered together with the correspondingAV-values. A value AV=AV 1 is determined such that the predeterminedrequirement is satisfied, for example such that the ratioZ_(min)/Z_(max) is as low as possible.

It is also possible to again perform the steps illustrated in FIG. 2 topossibly set a new ΔT if this is necessary in order to further improvethe cardiac output.

In the embodiment discussed above, the impedance value was sensedbetween electrodes 31, 32 of the first lead and electrodes 41, 42 of thesecond lead. However, it is also possible to sense other impedancevalues in the heart as for example illustrated in the above-discloseddocuments. For example, it is possible to sense an impedance valuebetween electrodes 51, 52 of the third lead 50 and electrodes 61, 62 ofthe fourth lead 60. Such an impedance value may be an indication of theamount of blood in the left atrium LA. It is also possible to use thisimpedance value in order to control the time interval ΔT and the timeinterval AV in the same manner as illustrated above.

The heart stimulating device 10 according to the invention thus has ahousing 12 with the control circuit 14 described above. However, theinvention also relates to a heart stimulating system. This systemincludes the heart stimulating device 10 and at least a first lead 30and a second lead 40 connected to the heart stimulating device 10 suchthat at least two electrodes 31, 41 are connected to the control circuit14. The system is preferably operates such that the measured impedancevalue Z is sensed between an electrode 31 or 32 of the first lead 30 andan electrode 41 or 42 of the second lead 40.

The invention also relates to a method for stimulating a heart usingsuch a heart stimulating system. According to this method, the firstlead 30 is arranged such that the first electrode 31 and/or 32 ispositioned to stimulate a first ventricle RV of a heart and the secondlead 40 is arranged such that the second electrode 41 and/or 42 ispositioned to stimulate the second ventricle LV of the heart. Accordingto his method of using the device the above-illustrated steps arecarried out.

The leads preferably are arranged such that the impedance value Z ismeasured across at least a part of one of said ventricles RV, LV,preferably across at least part of the left ventricle LV as has beendiscussed above.

1-16. (canceled)
 17. An implantable heart stimulating device comprising:a housing adapted for implantation in a subject; a pulse generatorcontained in said housing for interacting with a first ventricle and asecond ventricle of a heart of the subject to stimulate said firstventricle and said second ventricle; sensing circuitry contained in saidhousing adapted to obtain sensed signals from two different positionsrelative to the heart; and a control circuit contained in said housingand connected to said pulse generator and to said sensing circuitry,said control circuit controlling said pulse generator to emit respectivestimulating pulses to the first and second ventricles within a sameheart cycle with a time interval between the respective pulses that isvariable by said control circuit, said control circuit receiving saidsensed signals from said sensing circuitry and deriving an impedancevalue therefrom indicative of an impedance between said two differentpositions, said control circuit determining a minimum value and amaximum value of said impedance value during said heart cycle anddetermining a relationship between said minimum value and said maximumvalue, said control circuit varying said time interval and monitoringsaid relationship over a plurality of heart cycles, and said controlcircuit setting said time interval for causing said relationship tosatisfy a predetermined requirement.
 18. An implantable heartstimulating device as claimed in claim 17, wherein said control circuitforms a ration of said minimum value and said maximum value as saidrelationship.
 19. An implantable heart stimulating device as claimed inclaim 18 wherein said predetermined requirement is that said ratio isminimized.
 20. An implantable heart stimulating device as claimed inclaim 17 wherein said control circuit forms a difference between saidminimum value and said maximum value in determining said relationship.21. An implantable heart stimulating device as claimed in clam 20wherein said control circuit forms a ratio between said minimum valueand said difference as said relationship.
 22. An implantable heartstimulating device as claimed in claim 21 wherein said predeterminedrequirement is that said ratio is minimized.
 23. An implantable heartstimulating device as claimed in claim 22 wherein said control circuitchanges said time interval in a first direction selected from the groupconsisting of increasing said time interval and decreasing said timeinterval, and monitors a change in said ratio when said time interval ischanged in said first direction and, if said ratio decreases, saidcontrol circuit further changing said time interval in said firstdirection until said predetermined requirement is satisfied.
 24. Animplantable heart stimulating device as claimed in claim 23 wherein saidcontrol circuit, if said ratio increases, changes said time interval ina direction opposite to said first direction, and further changes saidtime interval in said opposite direction until said predeterminedrequirement is satisfied.
 25. An implantable heart stimulating device asclaimed in claim 17 further comprising an activity sensor adapted tointeract with the subject to obtain an activity signal indicative of alevel of physical activity of the subject, and wherein said controlcircuit varies said time interval and monitors said relationship over aplurality of heart cycles and sets said time interval for causing saidrelationship to satisfy said predetermined requirement, when saidactivity signal indicates said subject is at a low level of physicalactivity.
 26. An implantable heart stimulating device as claimed inclaim 17 wherein said pulse generator is also adapted to interact withan atrium of the heart to deliver stimulation pulses to the atrium, andwherein said control circuit controls delivery of respective stimulatingpulses delivered to the atrium and to said ventricles with a variableatrial-ventricular delay, and wherein said control circuit, whilemaintaining said time interval at the set value, varies saidatrial-ventricular delay and monitors said relationship over a pluralityof heart cycles, and sets said atrial-ventricular delay for causing saidrelationship to satisfy said predetermined requirement.
 27. Animplantable heart stimulating system comprising: a housing adapted forimplantation in a subject; a pulse generator contained in said housingfor interacting with a first ventricle and a second ventricle of a heartof the subject to stimulate said first ventricle and said secondventricle; sensing circuitry contained in said housing adapted to obtainsensed signals from two different positions relative to the heart; and acontrol circuit contained in said housing and connected to said pulsegenerator and to said sensing circuitry, said control circuitcontrolling said pulse generator to emit respective stimulating pulsesto the first and second ventricles within a same heart cycle with a timeinterval between the respective pulses that is variable by said controlcircuit, said control circuit receiving said sensed signals from saidsensing circuitry and deriving an impedance value therefrom indicativeof an impedance between said two different positions, said controlcircuit determining a minimum value and a maximum value of saidimpedance value during said heart cycle and determining a relationshipbetween said minimum value and said maximum value, said control circuitvarying said time interval and monitoring said relationship over aplurality of heart cycles, and said control circuit setting said timeinterval for causing said relationship to satisfy a predeterminedrequirement.
 28. A heart stimulating system as claimed in claim 27wherein said first lead carries said first sensing electrode and whereinsaid second lead carries said second sensing electrode.
 29. A method forstimulating a heart comprising the steps of: a housing adapted forimplantation in a subject; a pulse generator contained in said housingfor interacting with a first ventricle and a second ventricle of a heartof the subject to stimulate said first ventricle and said secondventricle; sensing circuitry contained in said housing adapted to obtainsensed signals from two different positions relative to the heart; and acontrol circuit contained in said housing and connected to said pulsegenerator and to said sensing circuitry, said control circuitcontrolling said pulse generator to emit respective stimulating pulsesto the first and second ventricles within a same heart cycle with a timeinterval between the respective pulses that is variable by said controlcircuit, said control circuit receiving said sensed signals from saidsensing circuitry and deriving an impedance value therefrom indicativeof an impedance between said two different positions, said controlcircuit determining a minimum value and a maximum value of saidimpedance value during said heart cycle and determining a relationshipbetween said minimum value and said maximum value, said control circuitvarying said time interval and monitoring said relationship over aplurality of heart cycles, and said control circuit setting said timeinterval for causing said relationship to satisfy a predeterminedrequirement.
 30. A method as claimed in claim 29 wherein the step ofsensing said impedance between said two positions comprises sensing saidimpedance across at least a portion of one of said first ventricle andsaid second ventricle.
 31. A method as claimed in claim 29 wherein thestep of sensing said impedance between said two positions comprisessensing said impedance across a left ventricle of the heart.
 32. Amethod as claimed in claim 29 wherein the step of sensing said impedancebetween said two positions comprises sensing said impedance across atleast a part of a left atrium of the heart.